At 17 I tried to throw a guy who was getting a bit rough while sparing doing karate (before my aikido life). Lovely throw until I realised I was going to drop him on his head and stopped mid throw, and he fell across my knee (because I didn't know what I was doing). Leg bend 90 degrees inward, snapped my ACL and damaged the meniscus, OUCH!!!!. Over 20 years later and the injury had developed into advanced osteoarthritis.